1. Field of the Invention
This invention relates to power converters, and more particularly, to high voltage, large capacity power converters which connect in series multiple self-turn-off switching devices.
2. Description of the Related Art
In recent years, power converters which can supply high voltages and large currents have been desired. In particular, the ability to supply 2000.about.3000 A or more at 100.about.500 KV is required by DC transmission and the like.
FIG. 1 is an example of the composition of a prior art power converter for high-voltage and large current.
In FIG. 1, AC power source 1 is connected to converter 3 via transformer 2. Converter 3 is composed by bridge-connected switch units 21.about.26. Switch unit 21 is composed of self-turn-off switching devices which are bridge-connected. Here, an example of the case of composition using gate turn-off thyristors (hereafter, simply "GTO") is described. DC reactor 4 smooths the DC output current of converter 3. The DC output of converter 3 is connected to DC circuit load 5 via DC reactor 4.
Switch unit 21 is composed of GTO 6, GTO 7, diode 8, diode 9 and capacitor 11. Since switch units 22.about.26 are of the same composition as switch unit 21, their descriptions are omitted. Converter 3 converts AC power and DC power between AC power source 1 and DC circuit 5 bidirectionally by a 3-phase bridge composed of switch units 21.about.26.
In this example, an arm consists of one switch unit. However, arms may be formed by executing an appropriate number of parallel connected switch units, DC connections or the like.
Next, in FIG. 2, the operation of switch unit 21 is described.
In FIG. 2(a), unit 21 is in the conducting (ON) state. The current is divided into parallel paths; a first path passing through diode 8 and GTO 6, and a second path passing through GTO 7 and diode 9. The flow of the current is shown by the arrows.
FIG. 2(b) shows the state in switch unit 21 immediately after GTO 6 and GTO 7 have turned OFF. The current flows by the path of diode 8, capacitor 11 and diode 9, and charges capacitor 11. When capacitor 11 is charged and the current becomes zero, switch unit 21 will be in the broken (OFF) state.
In FIG. 2(c), switch unit 21 is in the above-mentioned broken (OFF) state.
FIG. 2(d) shows the state immediately after GTO 6 and GTO 7 have turned ON. The current flows by the path of GTO 7, capacitor 11 and GTO 6, and discharges the charge accumulated in capacitor 11. When capacitor 11 is discharged, diode 8 and diode 9 turn ON, and the current becomes in the state shown in FIG. 2(a).
Thereafter, the states in FIG. 2(a).about.FIG. 2(d) are repeated.
With the above type of power converter, there was the problem that the balance of the two parallel sharing currents in FIG. 2(a) (that is to say, the current sharing between each element) could not be maintained due to differences in dynamic characteristics, such as the switching characteristics of the GTO and diode circuit-composition elements, and static characteristics, such as forward voltage-drop, during transitions such as from the state in FIG. 2(d) to FIG. 2(a). If the current sharing cannot be maintained, the current in either of the two GTOs or diodes is biased, and the loss in that GTO or diode becomes greater. Ultimately, this is due to dependence on the randomness of the element.
When designing a power converter, the system must be designed taking this unbalance into consideration beforehand. In the end, the poorer the utilization factors of the elements, the higher becomes the cost of the system.